PROJECT SUMMARY/ABSTRACT The HIV epidemic in China is now concentrated among men who have sex with men (MSM), who account for a third of new HIV infections. Despite strengthened prevention efforts and increased availability of HIV testing in the country, testing rates are low and condomless sex is common among Chinese MSM. Globally, condomless sex among MSM, particularly with primary partners, is also ubiquitous. To reduce HIV transmission risks in the context of condomless sex, MSM communities have adopted a range of sexual harm reduction strategies (i.e., seroadaptive behaviors). Essential to the effectiveness of these strategies is accurate knowledge of HIV status within the partnership. This necessitates frequent HIV testing and mutual HIV status disclosure. However, significant proportions of MSM have never tested or not recently tested and are unaware of their own and their partner's HIV status, especially in settings where sexual minorities are stigmatized. Among Chinese MSM, their ability to successfully use these harm reduction strategies is severely limited by low levels of HIV testing and disclosure efficacy. As a result, many Chinese MSM make unreliable assumptions about their own and their partners' serostatus (i.e., seroguessing), resulting in risky condomless sex. As many of these men do not access facility-based testing, new and emerging HIV testing options must be expanded to help these men make informed decisions about sexual risk and harm reduction. HIV self-testing (HIVST) offers a promising prevention strategy to reach more untested Chinese MSM, increase testing frequency, and serve as a valuable prevention tool to assist harm reduction. As a natural extension of our previous research in the US and China, we propose to determine the feasibility and preliminary efficacy of an HIVST intervention in facilitating sexual harm reduction and reducing HIV transmission among MSM in Nanjing, China. We will: 1) conduct participant observation (N = 20) and cognitive interviews (N=10) to determine what information and support is needed for MSM to use HIVST safely and practice sexual harm reduction responsibly; and 2) recruit and enroll 300 high- risk HIV-negative MSM into a RCT where the intervention group will be offered HIVST kits plus harm reduction education and followed over a 9-month period to determine whether the intervention increases uptake and frequency of testing in partnership, improves disclosure and awareness of partners' HIV status, and therefore facilitates sexual harm reduction and reduces HIV transmission. Expanding HIV testing among key populations is a priority for HIV prevention worldwide. Innovative strategies are needed to encourage MSM to seek HIV testing while taking into consideration existing paradigms of community-originated and driven harm reduction strategies. As HIVST is being introduced to different settings and becoming more accessible, findings from this proposed study will provide vital information on current patterns of seroadaptation and HIVST use; identify what is needed to promote HIVST's proper use for harm reduction and linkage to HIV/STI care, and whether HIVST can improve the accuracy of harm reduction strategies.